Ask An Expert with Doug Marshall: The Effect of Preservation Aids on Pathogen Survival in Smoked Salmon
Topic: The Effect of Preservation Aids on Pathogen Survival
Customer Question:
“I have a cold smoked salmon product, to which the only added ingredient is salt. Is bacterial control affected only by salt extracting water and reducing the water activity (Aw) or does the presence of salt have its own bactericidal or bacterial control effect?"
Response by Douglas Marshall, Ph.D., CFS - Chief Scientific Officer
“Salt, when ionized in an aqueous environment, yields sodium ions that carry a positive charge and chloride ions with a negative charge. Water is a dipole molecule, the two hydrogens carry a slight positive charge and the oxygen carries a slight negative charge. So, water hydrogens (can be from many individual water molecules) will be attracted to chloride and water oxygen (again from many water molecules) will be attracted to sodium. The force of these attractions is so strong that water may become immobilized in a shell around each sodium and chloride ion. Immobilized water cannot serve as a solvent to allow enzyme reactions and some chemical reactions to occur. Such reactions are necessary for biological function, such as microbial growth. Sugars and polysaccharides (like pectin, starch, gums) also are able to immobilize water because they have a ring of hydroxyls (-OH), where the oxygen (O) again attracts water hydrogens and the hydrogen (H) attracts water oxygens. So, you’re not removing water by dehydration, you’re simply binding what water is in the matrix and making it unavailable to act as a solvent.
A microbial cell wall is permeable to water. There is some salt (and other solutes) inside the cell. Due to osmotic pressure, if there is more salt on the outside of the cell wall than inside, water will migrate out of the microbial cell to balance osmotic pressure. If too much water migrates outside the cell, then there will not be enough water inside to act as a solvent to support enzyme reactions and the cell will struggle to grow or survive. In contrast, if there is more salt inside the cell than outside, water will migrate inside and potentially create so much pressure inside to cause the cell wall to burst. This would be a lethal event, but difficult to accomplish since most environments have some solutes dissolved in the water phase. However, it can be accomplished by placing microbial cells in distilled or deionized water where there are little dissolved solutes.
Water activity is a measure of the ability of water in a matrix to act as a solvent. Water content is a measure of the amount of water in a matrix, with no consideration to whether such water is able to act as a solvent. For example, seawater (lower water activity) and fresh water (higher water activity) are both mostly water, but seawater contains enough salt to be lethal to humans if that is all we have to drink. The salt in seawater will draw water out of our tissue cells to balance the osmotic pressure. This will result in those cells, tissues, and organs to cease functioning resulting in death.
Both modes of action are at play when salt is used to control microbial growth in a food. Microbes have a wide range of sensitivity or resistance to salt and low water activity. For example, some halophiles (salt loving microbes) can tolerate a large amount of salt (low water activity environments) compared to others. Fungi (yeasts and molds) generally have a greater tolerance to low water activity environments than most bacteria.
In conclusion, to answer your question on whether salt is able to kill a microbe, you’ll need to perform a challenge study to definitively answer the question as every food and process is different so it is difficult to predict survival. It is easier to predict growth prevention and the FDA Fish and Fishery Products Hazards and Controls Guide has data on the minimum water activity for growth of each pathogen.”
Microbial Challenge Study Testing with Eurofins
When designing a microbial challenge study, a variety of factors must be considered, such as the intrinsic characteristics of the product formulation, storage conditions, the target organism(s), the inoculum preparation method, inoculation method and level, sampling considerations, and interpretation of test results. As each of these considerations is specific to the product, process, and overall study objective, it is important that the study is designed and evaluated by an expert food microbiologist. Eurofins Microbiology Laboratories, Inc. (Fresno, CA) has the breadth and depth of expertise required to design, execute, analyze, and interpret microbial challenge studies.